The Sea: Earth's Defining Feature — Salty, Dynamic, Alive

The sea covers 71% of Earth's surface (361M km²), holds 97% of its water, and drives the planetary systems that make life possible.

1. Salinity: The Signature

• Average 35 PSU (35 g/kg). Main ions: Na⁺, Cl⁻, Mg²⁺, SO₄²⁻, Ca²⁺, K⁺.
• Sources: Riverine weathering input + hydrothermal vents. Sinks: evaporites, biogenic uptake, subduction.
• Constant composition (Forchhammer's Principle) — ratios fixed, total varies.

2. Temperature: Latitude & Depth

• Surface: -2°C (polar) → 30°C (tropics).
• Thermocline: Rapid transition (100–1,000 m).
• Deep: 0–4°C globally (Antarctic Bottom Water).
• Heat capacity: 1,000× atmosphere → climate flywheel.

3. Waves: Wind Energy Made Visible

• Generated by wind fetch, duration, speed.
• Swell: Organized, long-period, travels 1000s km.
• Breaking: Shallow water → energy dissipation, sediment transport.
• Rogue waves: Nonlinear focusing — 20–30 m, once myth, now measured.

4. Currents: The Global Conveyor

• Wind-driven (surface): Gyres (5 subtropical), Ekman transport, western boundary currents (Gulf Stream, Kuroshio, Agulhas) — fast, warm, narrow.
• Thermohaline (deep): Density-driven (T/S). NADW formation → AABW → global overturning (1,000 yr cycle).
• Transport heat, nutrients, carbon, larvae, plastics.

5. Tides: Astronomical Forcing

• Moon (68%) + Sun (32%) gravity.
• Semidiurnal (2/day) dominant; diurnal (1/day) in some basins.
• Spring (syzygy) vs. neap (quadrature) cycle 14.8 days.
• Amplification in funnels (Bay of Fundy: 16 m range).

6. Life: The Majority of Earth's Biosphere

• Phytoplankton: 50% global photosynthesis.
• Habitats: Coral reefs, kelp forests, vents, seeps, abyssal plains.
• Fisheries: 170M tons/yr — 3B people depend on seafood protein.

7. Human Stakes

• Shipping: 90% global trade by volume.
• Energy: Oil, gas, wind, tidal, OTEC.
• Threats: Warming, acidification, deoxygenation, plastic, overfishing, noise.

The sea isn't a backdrop. It's the operating system of the planet.